In satellite communication systems which provide telecommunication links of high quality for fixed radio communications and for mobile radio communications utilized in ships and aircrafts, since the satellite-borne power sources have their own limits, there usually is adopted a device for heightening antenna gains rather than increasing transmitter outputs. The high-gain antennas utilizing spot beams, however, have sharp directivity and consequently are restricted to narrow service areas. Thus, multibeam antennas offering wider service areas by use of multiple spot beams have come to be adopted.
The multibeam antennas come in various types. For use in mobile-satellite communications, multibeam array antennas which can have their cross-over levels heightened by narrowing the spaces separating the adjacent beams are suitable. For the multibeam array antenna, the beam forming network which serves to distribute signals from the individual antenna elements, provide proper phase shift, and synthesize a multibeam constitutes itself an important device.
Butler matrix, Blass feed, and resistive matrix are examples of heretofore well-known beam forming networks available for the multibeam array antenna. The Butler matrix offers the merit that it can be composed of lossless circuits. It nevertheless has a disadvantage in that it permits no free beam formation because it entails a definite relation between the beam width and the spaces separating the adjacent beams. When the Butler matrix is applied for the planar array antenna, it becomes complicated and massive, and, therefore, is not suitable for installation on a satellite. In contrast, the resistive matrix can be constructed rather compactly with microstrip circuits and, therefore, proves particularly convenient for installation on a satellite.
The resistive coupling matrix is composed of a quadrature phase splitter serving to divide input signals into signals of four phases each of 90.degree. and a group of coupling resistors serving to connect the lines from the phase splitter from which signals involving a phase difference of 90.degree. are outputted with corresponding output summing lines. By suitably selecting signals involving a phase difference of 90.degree. and resistance values of coupling resistors, this network enables signals of desired phases to be selectively fed into the output summing lines. This beam forming network can be applied to array antennas having antenna elements in arbitrary arrangements and has the outstanding advantage that multiple beams can be formed in desired directions. Moreover, it can be constructed rather simply such as with microstrip lines, for example. This resistive coupling matrix, therefore, serves particularly advantageously as a beam forming network for the multibeam antenna to be borne on a satellite. It is, however, generally used in the intermediate frequency band because the network is susceptible to loss. Examples of the beam forming network using a quadrature phase splitter and coupling resistors are found in the dissertation titled "Fixed beam forming, in `Phased Array Radar Studies, 1 July 1959 to 1 July 1960`" written by S. Spoerri and published in Part 2, Chapter 4, Technical Report No. 228 (1960), MIT Lincoln Laboratory and in the dissertation titled "Multibeam Generation at L-Band: A Phased-Array Approach," written by R. Coirault and W. Kriedte and published in European Space Agency Journal, 1980, Vol. 4, pp 319-336.
When symmetry exists in the arrangement of elements and/or the arrangement of beams, the input signals and/or output signals of symmetrically arranged elements and/or beams are disposed mutually complex conjugates. This fact evinces the requirement that the phase shifts which the beam forming network ought to give should also have the relationship of mutually complex conjugates respective to symmetrical arrangements of elements and/or beams.
Moreover, it should be noted that the number of coupling points involved in this matrix is 2.times.M.times.N on the maximum, wherein N stands for the number of antenna elements and M the number of beams. Further, the number of antenna elements generally increases proportionally with the increasing number of beams. In the multibeam antenna which has a large number of beams, therefore, the matrix becomes notably bulky and consequently entails various difficulties structurally and also from the standpoint of electrical properties. This beam forming network is inherently intended for application to multibeam antennas involving arbitrary arrangements of antenna elements and beams. It has been heretofore applied in its unaltered form to multibeam antennas wherein practically important arrangements of antenna elements and beams involve symmetry. Owing to the problems issuing from the bulky structure of the matrix mentioned above, it has found utility in a limited range of applications.